1. Field of the Invention
The present invention relates to an ultrasonic transmitting and receiving apparatus for transmitting ultrasonic waves and receiving echo signals, thereby obtaining ultrasonic images.
2. Description of a Related Art
Conventionally, as an element used for transmitting and receiving ultrasonic waves, that is, an ultrasonic transducer, a piezoelectric element that includes piezoelectric ceramic represented by PZT (Pb (lead) zirconate titanate) or macromolecule piezoelectric material represented by PVDF (polyvinylidene difluoride) has been generally used.
FIG. 13 shows a structure of a transducer included in an ultrasonic probe that is generally used in an ultrasonic imaging apparatus and an acoustic field distribution of an ultrasonic beam transmitted from the transducer. As shown in FIG. 13, the ultrasonic transducer array 100 is fabricated by, for example, linearly arranging a number of piezoelectric elements 101 on ends of which electrodes 102 and 103 are formed. A drive signal generating circuit including a pulser etc. is connected to the electrodes 102 and 103. Applying a voltage to such piezoelectric elements via the electrodes, the piezoelectric elements expand and contract by a piezoelectric effect to generate ultrasonic waves. By driving plural piezoelectric elements at predetermined time intervals, spherical waves transmitted from the respective piezoelectric elements are synthesized and a focus F of the ultrasonic beam can be formed in a desired direction and a desired depth.
The acoustic field of the ultrasonic wave formed as described above can be defined by an angle Ω in anticipation of the focal position from an aperture of the ultrasonic transducer array 100 and a directivity angle θ determined by an aperture of a transducer. That is, as shown in FIG. 13, the ultrasonic wave transmitted from the ultrasonic transducer array 100 converges once around the focus F and is diffused again. Further, in front and back of the focus F, a narrow beam zone “h” is shown.
By the way, in order to improve image quality of an ultrasonic image, it is necessary to improve resolving power when transmitting and receiving the ultrasonic beams. Under present circumstances, axial resolving power depending on ultrasonic frequency is sufficient, but lateral resolving power depending on beam width (beam diameter) is not yet sufficient. To improve the lateral resolving power, the beam is required to be narrowed. However, as clearly understood from FIG. 13, the narrow beam zone is short in the conventional beam focusing technique, and therefore, a zone in which the lateral resolving power is good is small. Furthermore, in the ultrasonic beam formed by the synthesis of spherical waves as described above, there occurred a problem that energy efficiency of the beam is very bad in the vicinity of the focus.
As a countermeasure against such problems, it is proposed to use a nondiffracting beam. The nondiffracting beam means a beam having an unchangeable section pattern regardless of the distance. As the nondiffracting beam, an X-wave (see U.S. Pat. No. 5,720,708) and a Bessel beam are known. The Bessel beam is an ultrasonic beam to be transmitted in distribution, amplitude, and phase expressed by Bessel function. The theory of the Bessel beam is held in an infinite aperture, however, it is shown that the nondiffracting depth can be ensured in a finite aperture to some degree (see J. Durnin, “Exact solutions for nondiffracting beams. I. The scalar theory”, J. Opt. Soc. Am. 1987, Vol. 4, No. 4, pp.651-654). Note that the nondiffracting depth means a depth that the nondiffracting beam can reach within medium without changing its waveform.
The nondiffracting beam propagates to the nondiffracting depth without diffusing energy. Therefore, in the case where the beam can be applied to the ultrasonic transmission and reception, a zone of a uniform beam width is maintained, and therefore, ultrasonic imaging can be performed in which improvements of the lateral resolving power, the energy efficiency and so on are realized. For example, in the Japanese Unexamined Patent Application Publication JP-A-9-299370, an ultrasonic probe for transmitting ultrasonic waves according to Bessel function is disclosed. In JP-A-9-299370, there is described that, by using a Bessel beam, a structure of an electric circuit can be made relatively simple, images with high resolution power and good quality can be obtained, and request for decrease in diameter can be accommodated. Alternatively, obtaining a two-dimensional or three-dimensional image by transmitting and receiving the nondiffracting beam (see Jian-yu Lu, “2D and 3D High Frame Rate Imaging with Limited Diffraction Beams”, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, Vol. 44, No. 4, JULY 1997, pp.839-856) and steering of the nondiffracting beam (see Jian-yu Lu, “A Study of Two-Dimensional Array Transducers for Limited Diffraction Beams”, IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, Vol. 41, No. 5, SEPTEMBER 1994, pp.724-739) are also described.
On the other hand, conventionally, in order to obtain a two-dimensional or three-dimensional image, for example, like B-mode scanning, an ultrasonic beam is transmitted one by one to scan an object to be inspected, and a two-dimensional or three-dimensional image is synthesized on the basis of the obtained image information. However, according to such method, time lag between frames is large, and therefore, images in different times will be synthesized. On this account, when imaging moving regions, the synthesized image is blurred. Especially, when observing a region moving hard such as a circulatory organ, real time environment of at least 30 frames per second is required.
As a countermeasure against such problems, a simultaneous multidirectional transmission and reception technique is under study (see U.S. Pat. No. 6,179,780 B1). However, since the beam focusing technique disclosed in U.S. Pat. No. 6,179,780 B1 uses conventional phase matching, the problem between the resolving power of the ultrasonic beam and the length of the narrow beam zone and the problem of the energy efficiency in the focus still remain.